I have one and use one if I know I'm not going to be in my car (or near an outlet) for a long time. It's pretty bulky. I didn't think I'd mind the bulk, but it DOES make the phone ugly as hell.
With my regular battery, I can usually go from about 6am to 4pm before desperately needing a charge. With the 2600mah one, I can go until bedtime (about 10-11pm). Not a massive increase, but an important one.
Still, it's ugly as hell. I wish they tapered it in more near the camera area. So much unnecessary bulk at the top.
Nokia 5160 > Ericsson T28 > Ericsson T39 > Ericsson T68 > Audiovox SMT5600 > Ericsson T610 > Ericsson w600 > iPhone 3G > Asus P527 > mWg Zinc II > HTC Touch Diamond > Sony Ericsson x10i > Samsung Nexus S > Motorola Droid RAZR > Samsung Galaxy Nexus
Palm IIIe > Sony Clie S300 > Sony Clie N610c > Sony Clie NR70v > Sony Clie TG50 > Sony Clie UX50 > Dell Axim x51v > iPod Touch > Nook Color > HP Touchpad
Yes. I'm OLD.